# Copyright 2020 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


import networkx
from networkx.algorithms.components import strongly_connected_components


def has_loop(edges, threshold=2):
    """check if a list of edges representing a directed graph contains a loop

    args:
        edges: list of edge sets representing a directed graph i.e. [(1, 2), (2, 1)]
        threshold: min number of nodes contained in loop

    returns:
        bool
    """
    g = networkx.DiGraph()
    g.add_edges_from(edges)
    return any(len(comp) >= threshold for comp in strongly_connected_components(g))
